1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, and more particularly, to a method of manufacturing a semiconductor device having a planar single crystal semiconductor film surface on an insulator.
2. Description of the Background Art
As a technique for forming a single crystal semiconductor layer on an insulator, SIMOX (Separation by Implanted Oxygen) is known. SIMOX is a method of obtaining a structure, in which oxygen ions are implanted into a single crystal semiconductor substrate to form an embedded insulator so that mutually separated semiconductor layers are formed.
As another technique for forming a single crystal semiconductor layer on an insulator, a melting recrystallization method is known. In this method, a non crystal semiconductor on an insulating film is heated and melted by a heater or is exposed to and melted by energy beam.
Among above-described techniques, the melting recrystallization method by energy beam irradiation has received much attention as a means for implementing a three dimensional element. Here, "the three dimensional element" has multiple integrated circuit layers with insulating layers disposed therebetween, and intended to realize greatly improved functions and a very high degree of integration as compared with a conventional two dimensional element which has been a single integrated circuit.
While the known energy beam for use in the melting recrystallization method includes large output laser or electronic beam, the method using laser is more applicable to forming a single crystal semiconductor layer on an insulator because it can be easily operated. To form a single crystal semiconductor layer using the melting recrystallization method by laser irradiation, it is necessary to control temperature distribution in the melted semiconductor to begin the recrystallization at a predetermined portion. One of such temperature distribution controlling methods is the anti-reflection film method.
Next, the melting recrystallization method using laser beam and utilizing the anti-reflection film method will be briefly described. Referring to FIG. 5A, an insulating film 52 is formed on a semiconductor single crystal substrate 51 made of silicon, for example. Next, an opening portion 53 is provided to be a seed portion at a predetermined portion of the insulating film 52.
Next, referring to FIG. 5B, a polysilicon film 54 is formed on the insulating film 52 by the CVD (Chemical Vapor Deposition) method. The opening 53 is then filled with the polysilicon film 54. Next, referring to FIG. 5C, an anti-reflection film 55 formed of silicon nitride film is formed by the CVD method on the polysilicon film 54, and patterned to be a predetermined form.
Referring to FIG. 6, the anti-reflection films 55 are provided like stripes at predetermined intervals. The laser beam 60 is irradiated onto the polysilicon film 54 and the anti-reflection films 55. The laser beam 60 is then scanned in the direction shown with the arrow in FIG. 6 along the longitudinal direction of the anti-reflection films 55, for example.
Referring to FIG. 5D, because the laser beam is absorbed more into a portion under the anti-reflection film 55 than a portion without anti-reflection film, the temperature therein is higher. Therefore, the region 541 where the silicon is melted extends from the region under the anti-reflection film 55 toward a portion at which the seed portion 53a is to be provided. As semiconductor layers melted by laser irradiation solifies starting at portions at lower temperature, the recrystallization always begins at portions between the anti-reflection films, and the portions under the anti-reflection films 55 solidify at the end, because the portions under the anti-reflection films 55 have higher temperature. With respect to the thermal conductivity, as a crystal semiconductor has the biggest value, the heat in solidification diffuses mainly toward the crystal semiconductor. As a result, when the seed portion 53a is provided between the anti-reflection films 55, the recrystallization always begins at the seed portion, and epitaxial growth having a seed of the single crystal semiconductor substrate 51 is produced. Thus, a single crystal semiconductor film having the same crystal orientation as that of the single crystal semiconductor 51 as a substrate is obtained on an insulator 52.
When such a single crystal semiconductor film is formed, the region between the anti-reflection films solidifies first and the region under the anti-reflection film solidifies later, since the layer under the anti-reflection film has a temperature higher than that of the region between the anti-reflection films. The higher the temperature of the melt is, the lower the surface tension is. Accordingly, a previously solidifying portion A forms a convex and a later solidifying portion B forms a concave in the single crystal semiconductor as shown by the broken line in FIG. 7. Therefore, the surface of the single crystal semiconductor film 56 has irregularity corresponding to the anti-reflection films 55 as shown in FIG. 7. As the width of the anti-reflection film 55 is about 5 .mu.m and the distance between the anti-reflection films is about 10 .mu.m, the convex or concave exists about every 15 .mu.m.
An actual measurement example of a surface irregularity of a single crystal semiconductor film formed in this way is shown in FIG. 8. In FIG. 8, the axis of abscissa denotes a distance in a direction vertical to the direction of extention of the anti-reflection film. The measurement has been performed in case where the thickness of the recrystallized semiconductor film on an insulating film is 0.55 .mu.m. FIG. 8 shows that the magnitude of the irregularity on the surface of the semiconductor film is about more than .+-.0.06 .mu.m. Furthermore, even immediately after forming a noncrystal or polycrystal film before recrystallization, there are convexes and concaves corresponding to the silicon grain 542 on the surface of the noncrystal or polycrystal film 54 as shown in FIG. 9. Such irregularity on the surface causes inconvenience in the various processes in device manufacturing and lack of uniformity in device performance. Although it is known that the device performance advances if the thickness of the semiconductor film is 0.1 .mu.m or less when a semiconductor element is formed in a semiconductor film on an insulating film, it is difficult to obtain a planar thin film even by etch back method which is common as planarization technology because of the above-described surface irregularity.
The reasons of the difficulty in thinning a film will be described below referring to FIG. 10. When the irregularity on the semiconductor film 56a is very small, it is possible to form a planar resist film 57a on the semiconductor film 56a. Accordingly, when etching is performed using the planar surface 571 as a reference plane, as long as the etching rate of the semiconductor film 56a is equal to that of the resist film 57a, a planar and thin semiconductor film can be obtained. On the other hand, referring to FIG. 11, when the irregularity on the semiconductor film 56b is of long cycle, the surface of the resist film 57b formed on the semiconductor film 56b also has the convex and concave corresponding to those on the semiconductor film 56b, so that even if etching is performed using the surface 572 of the resist film 57b as a reference plane, a planar surface can not be obtained.
As a method for reducing the above-described surface irregularity, polishing method may be applicable. Especially, so-called rigid body polishing method using rigid body such as SiO.sub.2 as a surface plate without a polishing pad may be suitable. However, as the thickness of the semiconductor film on an insulating film is originally about no more than 0.55 .mu.m, polishing it intactly by the rigid body polishing method makes the surface rough, so that a uniform and plane surface can not be obtained.
A section and plan view of a surface of a sample after polished are shown in FIG. 12. In FIG. 12, a number of scratches 71 are formed in the surface region 70 on the insulating film 52. In the region 72, the semiconductor film is absent. The surface becomes rough as shown in FIG. 12 because the semiconductor film is so thin that it is peeled off in polishing. As a method of forming a single crystal semiconductor film on an insulating film, as described above, the laser recrystallization method is employed when it is applied to implementing three dimensional elements. In this case, it is most suitable to obtain a crystal film with thickness of about 0.55 .mu.m by recrystallization. It is difficult to obtain good crystals when it thicker or thinner. Therefore it is difficult to have a thick semiconductor film.